This invention relates to a coordinate measuring system with a measuring head that can travel on a carriage on a guide unit along a scale and with a scanner for measuring a workpiece, the scanner being mounted on a rotating body which is arranged to be pivotable about an axis on a carriage which can travel along the guide unit.
As a result of new industrial manufacturing processes and more stringent requirements with regard to production measurement techniques used therein, coordinate measuring apparatus are being used with increasing frequency on production lines. A new measurement technique based on a photogrammetric measuring principle has been developed which makes it possible to perform precision measurements even outside of any fine measurement room. This technique is described in U.S. Pat. No. 5,017,013. The basis of the new method for measuring length is a photogrammetric principle which permits simultaneous determination of two coordinates by a single recorder. In this method, the quality of the measurements does not depend on the precision of the track on which the recorder travels.
The known apparatus essentially comprises a scale of lengths and a camera. During the measurement procedure, the marks on the scale are projected by an optical system on a CCD cell. The position and height of the focal center of the camera are determined from the measurement signals by means of a computer. These position and level determinations are performed simultaneously for each measurement time point, The use of special coding of the lines on the scale makes possible an absolute determination of the position coordinate. Thus, the possibility of cumulative error is excluded. Using this principle of measurement it is possible to create a measuring system with an absolute scale, which simultaneously supplies a coordinate in the lengthwise direction and a coordinate at right angles to the scale surface. The simultaneous detection of the second coordinate makes it possible through the use of a computer to measure inaccuracies and wear on guides. Thus, the precision of measuring and production machines becomes independent of the quality of the mechanical guiding means. Machines equipped in this manner are capable of compensating for their own errors.
On the other hand, U.S. Pat. No. 5,131,166 discloses a coordinate measuring apparatus with a measuring head which can travel on a carriage on a guide unit along a scale, and with a scanner for measuring a workpiece, in which the scanner is mounted on a rotating body which is arranged to be pivotable about an axis on a carriage which can travel on the guide unit. The description relating to FIG. 10 of this patent furthermore describes an additional selsyn for measuring the rotation in an additional plane. It is disadvantageous that the fine positioning is performed exclusively by a known selsyn because the accuracy achievable in this case is only in the micrometer range.
U.S. Pat. No. 5,134,782 also describes a measuring system with a rotating scanner. Again in this apparatus, the evaluation is performed by means of a known rotational position indicator or selsyn.
Finally, U.S. Pat. No. 4,587,622 discloses a method and apparatus for determining and correcting guidance errors. In this case the swinging movements are small, and the swinging movement is determined with two incremental linear pickups. Here too an accuracy of only about 3 micrometers can be achieved, which can be approximated by using a compensation polynomial of the 8th degree with a bandwidth of .+-.0.4 .mu.m.